The present invention relates to a process for producing a photographic support which comprises coating a polyolefin resin on both sides of a paper used as a base sheet (the paper of the base sheet is hereinafter referred to as "base paper"). In particular, it relates to a process for producing a photographic support which is free from surface defects and excellent in surface smoothness.
Photographic supports other than those for photosensitive film materials which have been commonly used are the so-called baryta papers formed by coating a baryta layer comprising mainly barium sulfate on one side of a paper. In recent years, however, with the aim of automation and the rapidity of development operations, water-resistant supports formed by coating both sides of the base paper with polyolefin resin, namely polyolefin resin-coated papers, have been used. However, polyolefin resin-coated paper has a number of problems when used as a photographic support.
In general, photographic polyolefin resin coated-paper is produced by using a melt-extrusion coating method. In this case, the polyolefin resin layer on the side to which a photographic emulsion is to be applied is incorporated with titanium dioxide pigment to improve the sharpness and resolution of photographic images. Production of such a polyolefin resin-coated paper containing titanium dioxide pigment in the resin layer involves various difficulties. First, polyolefin resin layers containing titanium dioxide pigment are highly susceptible to oxidative degradation. Since the decomposition and oxidation products of resin adversely affect on photographic emulsion, extrusion coating needs to be conducted at as low a temperature as possible, which, however, results in very poor adhesion between the base paper and the polyolefin resin layer. Secondly, polyolefin resin incorporated with titanium dioxide is expensive. This is because the cost of titanium dioxide pigment and the expense involved in incorporating the titanium dioxide pigment into the resin are added to the cost of resin. This necessitates reducing the amount of polyolefin resin incorporated with titanium dioxide pigment to be used, which results in the deterioration of sharpness and resolution.
Furthermore, photographic polyolefin resin-coated paper have other problems. Namely, whereas a high level of smoothness is required in a photographic polyolefin resin-coated paper having a glossy surface, the surface smoothness of resin-coated paper is very unsatisfactory particularly when the thickness of the polyolefin resin layer of the photographic emulsion coating side (hereinafter abbreviated as "front side") formed by extrusion-coating a molten polyolefin resin on base paper is 40 .mu.m or less. First, the resin surface of the front side of resin-coated paper has a frizzy, uneven or grainy appearance affected by the texture of base paper, resulting in the deterioration of specular glossiness. Second, pit-formed fine pores develop on the resin surface of the front side of resin-coated paper. Development of such pit-formed fine pores lowers the gloss of the glossy surface of the developed photographic printing paper comprising the resin-coated paper as the support, which greatly lowers the market value of the paper.
With the objective of obviating the various problems and additional difficulties of the photographic polyolefin resin-coated paper mentioned above, methods of double layer extrusion coating based on coextrusion coating or stepwise extrusion coating have been disclosed or exemplified in Japanese Patent Application Kokoku (Post-Exam. Publn.) No. 49-30446 (1974) and Japanese Patent Application Kokai (Laid-Open) Nos. 56-62248 (1981) and 59-198451 (1984). However, these methods are unsatisfactory for solving the above-mentioned problems and, in particular, very unsatisfactory for preventing the development of pit-formed fine pores on the front side of photographic polyolefin resin-coated paper.